1. Field of the Invention
This invention relates generally to methods and apparatus for drilling wells, particularly wells for the production of petroleum products, and more specifically concerns an actively controlled rotary steerable drilling system that can be connected directly to a rotary drill string or can be connected in a rotary drill string in assembly with a mud motor and/or thruster and/or flexible sub to enable drilling of deviated wellbore sections and branch bores. This invention also concerns methods and apparatus enabling precision control of the direction of a wellbore being drilled. This invention also concerns an actively controlled rotary steerable drilling system incorporating a hydraulically energized bit shaft positioning mechanism for accomplishing automatic geostationary positioning of the axis of an offsetting mandrel and drill bit during rotation of the offsetting mandrel and drill bit by a rotary drill string, mud motor or both. This invention further concerns elongate elastic anti-rotation blades projecting radially from the sliding tool collar for maintaining anti-rotation of the drilling tool with the borehole wall.
2. Description of Related Art
An oil or gas well often has a subsurface section that is drilled directionally, i.e., inclined at an angle with respect to the vertical and with the inclination having a particular compass heading or azimuth. Although wells having deviated sections may be drilled at any desired location, such as for "horizontal" borehole orientation or deviated branch bores from a primary borehole, for example, a significant number of deviated wells are drilled in the marine environment. In such case a number of deviated wells are drilled from a single offshore production platform in a manner such that the bottoms of the boreholes are distributed over a large area of a producing horizon over which the platform is typically centrally located, and wellheads for each of the wells are located on the platform structure.
In circumstances where the well being drilled is of complex trajectory, the capability provided by the rotary steerable drilling system of this invention to steer the drill bit while the drill bit is being rotated by the collar of the tool enables drilling personnel to readily navigate the wellbore being drilled from one subsurface oil reservoir to another. The rotary steerable drilling tool of the present invention enables steering of the wellbore both from the standpoint of inclination and from the standpoint of azimuth so that two or more subsurface zones of interest can be controllably intersected by the wellbore being drilled.
A typical procedure for drilling a directional borehole is to remove the drill string and drill bit by which the initial, vertical section of the well was drilled using conventional rotary drilling techniques, and run in a mud motor having a bent housing at the lower end of the drill string which drives the bit in response to circulation of drilling fluid. The bent housing provides a bend angle such that the axis below the bend point, which corresponds to the rotation axis of the bit, has a "toolface" angle with respect to a reference, as viewed from above. The toolface angle, or simply "toolface", establishes the azimuth or compass heading at which the deviated borehole section will be drilled as the mud motor is operated. After the toolface has been established by slowly rotating the drill string and observing the output of various orientation devices, the mud motor and drill bit are lowered, with the drill string non-rotatable to maintain the selected toolface, and the drilling fluid pumps, "mud pumps", are energized to develop fluid flow through the drill string and mud motor, thereby imparting rotary motion to the mud motor output shaft and the drill bit that is fixed thereto. The presence of the bend angle causes the bit to drill on a curve until a desired borehole inclination has been established. To drill a borehole section along the desired inclination and azimuth, the drill string is then rotated so that its rotation is superimposed over that of the mud motor output shaft, which causes the bend section to merely orbit around the axis of the borehole so that the drill bit drills straight ahead at whatever inclination and azimuth have been established. If desired, the same directional drilling techniques can be used as the maximum depth of the wellbore is approached to curve the wellbore to horizontal and then extend it horizontally into or through the production zone. Measurement-while-drilling "MWD" systems are commonly included in the drill string above the mud motor to monitor the progress of the borehole being drilled so that corrective measures can be instituted if the various borehole parameters indicate variance from the projected plan.
Various problems can arise when sections of the wellbore are being drilled with the drill string non-rotatable and with a mud motor being operated by drilling fluid flow. The reactive torque caused by operation of a mud motor can cause the toolface to gradually change so that the borehole is not being deepened at the desired azimuth. If not corrected, the wellbore may extend to a point that is too close to another wellbore, the wellbore may miss the desired "subsurface target", or the wellbore may simply be of excessive length due to "wandering". These undesirable factors can cause the drilling costs of the wellbore to be excessive and can decrease the drainage efficiency of fluid production from a subsurface formation of interest. Moreover, a non-rotating drill string may cause increased frictional drag so that there is less control over the "weight on bit" and the rate of drill bit penetration can decrease, which can result in substantially increased drilling costs. Of course, a non-rotating drill string is more likely to get stuck in the wellbore than a rotating one, particularly where the drill string extends through a permeable zone that causes significant build up of mud cake on the borehole wall.
A patent related to the subject matter of the present invention is U.S. Pat. No. 5,113,953. The '953 patent presents a directional drilling apparatus and method in which the drill bit is coupled to the lower end of a drill string through a universal joint, and the bit shaft is pivotally rotated within the steerable drilling tool collar at a speed which is equal and opposite to the rotational speed of the drill string. The present invention is significantly advanced as compared to the subject matter of the '953 patent in that the angle of the bit shaft or mandrel relative to the drill collar of the present invention is variable rather than being fixed. Additionally, the rotary steerable drilling system of the present invention incorporates various position measurement systems and position signal responsive control. Other patents of interest related to the present invention are UK Patents GB 2 177 738 B, GB 2 172 324 B and GB 2 172 325 B. The '738 patent is entitled "Control of drilling courses in the drilling of boreholes" and discloses a control stabilizer 20 having four actuators 44. The actuators are in the form of flexible hoses or tubes which are selectively inflated to apply a lateral force to the drill collar as shown at 22 for the purpose of deflecting the drill collar and thus altering the course of the borehole being drilled. The '324 patent is of interest to the present invention in that it presents a steerable drilling tool having stabilizers 18 and 20, with a control module 22 located between them for effecting controlled deflection of the drilling tube 10 for altering the course of the wellbore being drilled. The '325 patent is of interest to the present invention in that it presents a steerable drilling tool having a stabilizer housing 31 that contains sensing means and is maintained essentially stationary during drilling by an anti-rotation device 40. Movement of the drilling tube 10 relative to a wall contact assembly 33 is accomplished by applying different pressures, in a controlled manner, to each of four actuators 44. Steering of the drill bit is accomplished by sensing direction responsive deflection of the drilling tube 10. In contrast, the present invention achieves steering of the drill bit by hydraulically maintaining an offsetting mandrel, to which the drill bit is attached, in geostationary position and oriented about a knuckle or pivot mount within a sliding tool collar while the offsetting mandrel is rotatably driven within the sliding tool collar.
The present invention is also distinguished from the teachings of the related art in the assembly of drilling system controllable mud motor and thruster apparatus and a flexible sub that can be arranged in any suitable assembly to enable directionally controlled drilling to be selectively powered by a rotary drill string, a mud motor, or both, and to provide for precision control of weight on bit and accuracy of drill bit orientation during drilling.
U.S. Pat. No. 5,265,682 presents a system for maintaining a downhole instrumentation package in a roll stabilized orientation by means of an impeller. The roll stabilized instrumentation is used for modulating fluid pressure to a set of radial pistons which are sequentially activated to urge the bit in a desired direction. The drill bit steering system of the '682 patent most notably differs from the concept of the present invention in the different means that is utilized for deviating the drill bit in the desired direction. Namely, the '682 patent describes a mechanism which uses pistons which react against the borehole wall to force the bit in a desired lateral direction within the borehole. In contrast, the rotary steerable drilling system of the present invention incorporates an automatically energized, sensor responsive hydraulic system to maintain the bit shaft of the drilling system in geostationary and angularly oriented relation with the sliding tool collar to keep the drill bit pointing in a desired borehole direction. The hydraulic bit shaft positioning system positions the bit shaft axis in its knuckle or universal joint support within the sliding tool collar in order to keep the bit shaft pointed in the desired direction. Within the scope of the present invention various position sensors and electronics of the tool are located within the sliding collar of the drilling tool, rather than in a rotating component, to ensure the accuracy and extended service life thereof.